1. Field of the Invention
The present invention relates to a superconducting material and a method for preparing the same. More particularly, it relates to a novel superconducting material composed of compound oxide having a high critical temperature and a method for preparing the same.
2. Description of the Related Art
The phenomenon of superconductivity is understood in the art as a condition wherein perfect diamagnetism is observed and no difference in potential is observed and wherein an electric current of a constant finite value is observed internally in the superconducting medium.
Superconductivity and superconducting materials can be utilized in the field of electrical power applications such as MHD power generation, fusion power generation, power transmissions, electrical power reservation or the like; in the field of transportation, for example, magnetic levitation trains, magnetically propelled ships or the like; in the medical field, for example, in a high-energy beam radiation unit; in the field of science, such as in NMR or high-energy physics; for highly sensitive sensors or detectors for sensing very weak magnetic fields, microwave, radiant rays or the like, or in the field of fusion power generation. In addition to the above mentioned electrical power applications, superconducting materials can be used in the field of electronics, for example, as a device using the Josphson device which is expected to be a high-speed and low-power-consuming switching device.
The phenomenon of superconductivity, however, has heretofore been observed only at very low cryogenic temperatures. In fact, a relatively low temperature of 23.2.degree. K., which was the critical temperature (Tc) of a superconductor composed of Nb.sub.3 Ge has until recently been the highest record critical temperature among known superconducting materials.
At such temperature, liquidized helium (boiling point of 4.2.degree. K.) is the only cryogen which can be employed to realize this very low critical temperature in the superconducting material. However, helium is not only a limited, costly resource, but also requires a large-scale system for liquefaction. Therefore, there had been a strong demand for other superconducting materials having higher critical temperatures (Tc), however no materials having critical temperatures exceeding the above mentioned Tc had been discovered in approximately the past ten years.
Reports of the existence of a new type of superconducting material having a much higher Tc were published by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189]. It has also been reported in February 1987 that C. W. Chu et al discovered, in the United States of America, another high-temperature superconducting material, termed a YBCO type, of the form YBa.sub.2 Cu.sub.3 O.sub.7-x, having a critical temperature of about 90.degree. K. Hence, much activity in the field of high-temperature superconductors has taken place in the ensuing months.
It had been known that certain compound oxide ceramic materials exhibited the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses a Ba-Pb-Bi type compound oxide which exhibits superconductive properties and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also exhibit superconductive properties. These superconducting materials, however, possess rather low transition temperatures of about 10.degree. K. and hence usage of liquidized helium (boiling point of 4.2.degree. K.) as a cryogen had been indispensable in order to cool the material sufficiently to realize superconductivity. Therefore, the above mentioned new type compound oxides in which superconductivity is realized at temperatures attainable using liquid nitrogen, which is a relatively cheap cryogen, will accelerate actual usage of superconductors.
An object of the present invention is to provide a new system of compound oxides which possess a higher critical temperature and a method for preparing the same.